Electro-Polymerized Coating for a Wet Electrolytic Capacitor

ABSTRACT

A wet electrolytic capacitor that contains a casing within which is positioned an anode formed from an anodically oxidized sintered porous body and a fluidic working electrolyte is provided. The casing contains a conductive coating disposed on a surface of a metal substrate. The casing contains a metal substrate coated with a conductive coating. The conductive coating contains a conductive polymer layer formed through anodic electrochemical polymerization (“electro-polymerization”) of a colloidal suspension on the surface of the metal substrate. The conductive coating also contains a precoat layer that is discontinuous in nature and contains a plurality of discrete projections of a conductive material that are deposited over the surface of the metal substrate in a spaced-apart fashion so that they form “island-like” structures.

BACKGROUND OF THE INVENTION

Electrolytic capacitors typically have a larger capacitance per unitvolume than certain other types of capacitors, making them valuable inrelatively high-current and low-frequency electrical circuits. One typeof capacitor that has been developed is a “wet” electrolytic capacitorthat includes a sintered tantalum powder anode. These tantalum slugsfirst undergo an electrochemical oxidation that forms an oxide layercoating acting as dielectric over the entire external and internalsurfaces of the tantalum body. The anodized tantalum slugs may then besealed within a metal casing (e.g., tantalum) containing a liquidelectrolyte solution. To enhance capacitance, a finely dividedconductive material is often applied to the metal casing that is formedfrom activated carbon or ruthenium oxide. Unfortunately, however, suchcoatings are expensive and can also become easily detached under certainconditions. As such, a need still exists for an improved wetelectrolytic capacitor.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a wetelectrolytic capacitor is disclosed that comprises an anode formed froman anodically oxidized sintered porous body, a fluidic workingelectrolyte, and a casing within which the anode and working electrolyteare positioned. The casing contains a conductive coating disposed on asurface of a metal substrate. The coating contains a discontinuousprecoat layer that contains a plurality of discrete projections of aconductive material deposited over the surface of the metal substrate ina spaced-apart fashion so that the projections cover from about 5% toabout 80% of the surface of the metal substrate. The coating alsocontains a conductive polymer layer that overlies the discontinuousprecoat layer, wherein the conductive polymer layer is formed byelectrolytic polymerization of a colloidal suspension that includes aprecursor monomer.

In accordance with another embodiment of the present invention, a methodfor forming a casing of a wet electrolytic capacitor is disclosed. Themethod comprises forming a discontinuous precoat layer on a surface of ametal substrate, the precoat layer containing a plurality of discreteprojections of a conductive material deposited over the surface of themetal substrate in a spaced-apart fashion so that the projections coverfrom about 5% to about 80% of the surface of the metal substrate. Acolloidal suspension is applied to the metal substrate that comprises aprecursor monomer. An electrode is placed in contact with the metalsubstrate, and a current feed is supplied to the electrode to induceelectrolysis and oxidative polymerization of the precursor monomer,thereby forming a conductive polymer layer that overlies the precoatlayer.

Other features and aspects of the present invention are described inmore detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof to one skilled in the art, is set forth moreparticularly in the remainder of the specification, including referenceto the accompanying figures, in which:

FIG. 1 is a schematic view of one embodiment of the wet electrolyticcapacitor of the present invention;

FIG. 2 is a perspective view of one embodiment of an anode that may beemployed in the wet electrolytic capacitor;

FIG. 3 is a side view of the anode of FIG. 2;

FIG. 4 is a top view of the anode of FIG. 2;

FIG. 5 is a top view of another embodiment of an anode that may beemployed in the wet electrolytic capacitor of the present invention;

FIG. 6 is a FESEM microphotograph (25 k magnification) showing thetantalum/palladium/PEDT structure of a sample formed in Example 1; and

FIG. 7 is a FESEM microphotograph (25 k magnification) showing thetantalum/PEDT structure of a sample formed in Example 2.

Repeat use of reference characters in the present specification anddrawing is intended to represent the same or analogous features orelements of the present invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention.

Generally speaking, the present invention is directed to a wetelectrolytic capacitor that contains a casing within which is positionedan anode formed from an anodically oxidized sintered porous body and afluidic working electrolyte. The casing contains a metal substratecoated with a conductive coating. The degree of surface contact betweenthe conductive coating and the surface of the metal substrate isenhanced in the present invention by selectively controlling the mannerin which the coating is formed. More particularly, the conductivecoating contains a conductive polymer layer formed through anodicelectrochemical polymerization (“electro-polymerization”) of a colloidalsuspension on the surface of the metal substrate. The use of a colloidalsuspension can improve the degree of surface coverage and overallconductivity of the coating, and allow oligomeric chains to growimmediately adjacent to the surface of the metal substrate, which canincrease robustness and mechanical stability.

To even further enhance the ability of the conductive polymer layer toremain adhered to the metal substrate, the conductive coating alsocontains a precoat layer that is discontinuous in nature and contains aplurality of discrete projections of a conductive material that aredeposited over the surface of the metal substrate in a spaced-apartfashion so that they form “island-like” structures. These discreteprojections can effectively roughen the surface of the metal substrate,thereby improving the ability of the conductive polymer layer to beadhered thereto. Due to its discontinuous nature, a substantial portionof the conductive polymer layer may also directly contact the metalsubstrate, which can reduce ESR and improve capacitance. In this regard,the surface coverage of the projections on the metal substrate may alsobe selectively controlled to help achieve the desired electricalperformance. For example, the surface coverage of the projections istypically from about 5% to about 80%, in some embodiments from about 10%to about 70%, and in some embodiments, from about 15% to about 60%. Theaverage size of the projections may likewise be from about 50 nanometersto about 500 micrometers, in some embodiments from about 1 to about 250micrometers, and in some embodiments, from about 5 to about 100micrometers.

Various embodiments of the present invention will now be described infurther detail.

I. Casing

A. Metal Substrate

The metal substrate may serve as a cathode for the capacitor and may beformed from a variety of different metals, such as tantalum, niobium,aluminum, nickel, hafnium, titanium, copper, silver, steel (e.g.,stainless), alloys thereof, composites thereof (e.g., metal coated withelectrically conductive oxide), and so forth. Tantalum is particularlysuitable for use in the present invention. The geometric configurationof the substrate may generally vary as is well known to those skilled inthe art, such as in the form of a foil, sheet, screen, container, can,etc. The metal substrate may form the all or a portion of casing for thecapacitor, or it may simply be applied to the casing. Regardless, thesubstrate may have a variety of shapes, such as generally cylindrical,D-shaped, rectangular, triangular, prismatic, etc. If desired, a surfaceof the substrate may be roughened to increase its surface area andincrease the degree to which a material may be able to adhere thereto.In one embodiment, for example, a surface of the substrate is chemicallyetched, such as by applying a solution of a corrosive substance (e.g.,hydrochloric acid) to the surface. Mechanical roughening may also beemployed. For instance, a surface of the substrate may be abrasiveblasted by propelling a stream of abrasive media (e.g., sand) against atleast a portion of a surface thereof.

If desired, a dielectric layer may be formed on the metal substrateprior to application of the conductive coating such that it ispositioned between the substrate and the coating. The thickness of thedielectric layer may be controlled within a certain range, such as fromabout 10 nanometers to about 500 nanometers, in some embodiments fromabout 15 nanometers to about 200 nanometers, in some embodiments fromabout 20 nanometers to about 100 nanometers, and some embodiments, fromabout 30 nanometers to about 80 nanometers. Within intending to belimited by theory, it is believed that acids often present in theworking electrolyte can undergo secondary reactions with the metalsubstrate (e.g., tantalum) at relatively high temperatures. The presenceof a relatively thick dielectric layer can therefore help to passivatethe metal substrate, and thereby minimize the likelihood that theworking electrolyte will react with the substrate to reduce itsconductivity and increase ESR. By ensuring that the thickness iscontrolled within the ranges noted above, however, the conductivity ofthe casing is not reduced to such an extent that the electricalproperties of the capacitor are adversely impacted.

A surface of the metal substrate (e.g., interior surface) may besubjected to a voltage to initiate anodic formation (“anodization”) ofan oxide film (dielectric layer) as described above. For example, atantalum (Ta) substrate may be anodized to form a dielectric layer oftantalum pentoxide (Ta₂O₅). Anodization may be performed by initiallyapplying an electrolyte to the metal substrate, such as by dipping thesubstrate into a bath that contains the electrolyte, and then applying acurrent. The electrolyte is generally in the form of a liquid, such as asolution (e.g., aqueous or non-aqueous), dispersion, melt, etc. Asolvent is generally employed in the electrolyte, such as water (e.g.,deionized water); ethers (e.g., diethyl ether and tetrahydrofuran);alcohols (e.g., methanol, ethanol, n-propanol, isopropanol, andbutanol); triglycerides; ketones (e.g., acetone, methyl ethyl ketone,and methyl isobutyl ketone); esters (e.g., ethyl acetate, butyl acetate,diethylene glycol ether acetate, and methoxypropyl acetate); amides(e.g., dimethylformamide, dimethylacetamide, dimethylcaprylic/capricfatty acid amide and N-alkylpyrrolidones); nitriles (e.g., acetonitrile,propionitrile, butyronitrile and benzonitrile); sulfoxides or sulfones(e.g., dimethyl sulfoxide (DMSO) and sulfolane); and so forth. Thesolvent may constitute from about 50 wt. % to about 99.9 wt. %, in someembodiments from about 75 wt. % to about 99 wt. %, and in someembodiments, from about 80 wt. % to about 95 wt. % of the electrolyte.Although not necessarily required, the use of an aqueous solvent (e.g.,water) is often desired to facilitate formation of an oxide. In fact,water may constitute about 1 wt. % or more, in some embodiments about 10wt. % or more, in some embodiments about 50 wt. % or more, in someembodiments about 70 wt. % or more, and in some embodiments, about 90wt. % to 100 wt. % of the solvent(s) used in the electrolyte.

The electrolyte is electrically conductive and may have an electricalconductivity of about 1 milliSiemens per centimeter (“mS/cm”) or more,in some embodiments about 30 mS/cm or more, and in some embodiments,from about 40 mS/cm to about 100 mS/cm, determined at a temperature of25° C. To enhance the electrical conductivity of the electrolyte, acompound may be employed that is capable of dissociating in the solventto form ions. Suitable ionic compounds for this purpose may include, forinstance, acids, such as hydrochloric acid, nitric acid, sulfuric acid,phosphoric acid, polyphosphoric acid, boric acid, boronic acid, etc.;organic acids, including carboxylic acids, such as acrylic acid,methacrylic acid, malonic acid, succinic acid, salicylic acid,sulfosalicylic acid, adipic acid, maleic acid, malic acid, oleic acid,gallic acid, tartaric acid, citric acid, formic acid, acetic acid,glycolic acid, oxalic acid, propionic acid, phthalic acid, isophthalicacid, glutaric acid, gluconic acid, lactic acid, aspartic acid,glutaminic acid, itaconic acid, trifluoroacetic acid, barbituric acid,cinnamic acid, benzoic acid, 4-hydroxybenzoic acid, aminobenzoic acid,etc.; sulfonic acids, such as methanesulfonic acid, benzenesulfonicacid, toluenesulfonic acid, trifluoromethanesulfonic acid,styrenesulfonic acid, naphthalene disulfonic acid,hydroxybenzenesulfonic acid, dodecylsulfonic acid,dodecylbenzenesulfonic acid, etc.; polymeric acids, such aspoly(acrylic) or poly(methacrylic) acid and copolymers thereof (e.g.,maleic-acrylic, sulfonic-acrylic, and styrene-acrylic copolymers),carageenic acid, carboxymethyl cellulose, alginic acid, etc.; and soforth. The concentration of ionic compounds is selected to achieve thedesired electrical conductivity. For example, an acid (e.g., phosphoricacid) may constitute from about 0.01 wt. % to about 5 wt. %, in someembodiments from about 0.05 wt. % to about 0.8 wt. %, and in someembodiments, from about 0.1 wt. % to about 0.5 wt. % of the electrolyte.If desired, blends of ionic compounds may also be employed in theelectrolyte.

A current is passed through the electrolyte to form the dielectriclayer. The value of voltage manages the charge (current multiplied bytime) and thereby the thickness of the dielectric layer. For example,the power supply may be initially set up at a galvanostatic mode untilthe required voltage is reached. Thereafter, the power supply may beswitched to a potentiostatic mode in which the voltage is held constantto ensure that the desired dielectric thickness is formed over thesurface of the metal substrate. Of course, other known methods may alsobe employed, such as pulse methods. Regardless, to help achieve thedesired thickness for the dielectric layer as noted above, the formingvoltage used during anodization, which is typically equal to the peakvoltage, is typically high, such as about 5 volts or more, in someembodiments about 7 volts or more, in some embodiments from about 10volts to about 25 volts, and in some embodiments, from about 12 to about22 volts. The voltage level may vary (e.g., increasing) or remainconstant within this range. The temperature of the anodizing solutionmay range from about 10° C. to about 200° C., in some embodiments fromabout 20° C. to about 150° C., and in some embodiments, from about 30°C. to about 100° C. The resulting dielectric layer may thus be formed onthe surface of the metal substrate as described above.

B. Conductive Coating

The conductive coating is disposed on a surface of the metal substrate(e.g., interior surface) to serve as an electrochemically activematerial for the capacitor and includes a precoat layer and conductivepolymer layer. Any number of layers may be employed in the coating. Forexample, the coating may contain one or multiple precoat layers and oneor multiple conductive polymer layers. Other layers may also be employedin the coating if so desired. While the particular arrangement of theindividual layers may vary, it is typically desired that at least oneprecoat layer overlies the metal substrate, and that at least oneconductive polymer layer overlies the precoat layer so that the precoatlayer is positioned between the conductive polymer layer and the metalsubstrate.

i. Precoat Layer

Any of a variety of different materials may generally be employed in thepresent invention to form the discontinuous precoat layer. For example,a particulate material may be employed that includes conductiveparticles, such as those formed from ruthenium, iridium, nickel,rhodium, rhenium, cobalt, tungsten, manganese, tantalum, niobium,molybdenum, lead, titanium, platinum, palladium, and osmium, as well ascombinations of these metals. Non-insulating oxide conductive particlesmay also be employed. Suitable oxides may include a metal selected fromthe group consisting of ruthenium, iridium, nickel, rhodium, rhenium,cobalt, tungsten, manganese, tantalum, niobium, molybdenum, lead,titanium, platinum, palladium, and osmium, as well as combinations ofthese metals. Particularly suitable metal oxides include rutheniumdioxide, niobium oxide, niobium dioxide, iridium oxide, and manganesedioxide. Carbonaceous particles may also be employed that have thedesired level of conductivity, such as activated carbon, carbon black,graphite, etc. Some suitable forms of activated carbon and techniquesfor formation thereof are described in U.S. Pat. No. 5,726,118 to Ivey,et al. and U.S. Pat. No. 5,858,911 to Wellen, et al.

In particularly suitable embodiments, the precoat layer may be formedfrom a noble metal that can further enhance the electrical performanceof the capacitor by helping to minimize electrochemical reactionsbetween components of the coating and the fluid electrolyte, which mightotherwise generate reactive hydrogen radicals that lead to embrittlementof the metal substrate and a degradation in performance (e.g., increaseequivalence series resistance (“ESR”), decrease capacitance, etc.). Moreparticularly, the present inventors believe that the noble metal can actas a catalyst to drive electrochemical reactions towards the formationof hydrogen gas, which is less reactive than hydrogen radicals and thusless detrimental to capacitor performance.

Suitable noble metals may include ruthenium, rhodium, palladium, silver,osmium, iridium, platinum, and gold. Noble metals of the platinum familyare particularly suitable for use in the present invention, such asruthenium, rhodium, palladium, osmium, iridium, and platinum. Any of avariety of known techniques may generally be employed to apply a noblemetal precoat layer to the metal substrate. Suitable methods mayinclude, for instance, electrolytic plating, vapor deposition,electroless plating, etc., such as described in U.S. Pat. No. 4,780,797to Libby and U.S. Pat. No. 3,628,103 to Booe. As is known in the art,for example, electroless plating may rely upon the presence of areducing agent (e.g., hydrated sodium hypophosphite) that can react withnoble metal ions to deposit a noble metal onto a surface of the metalsubstrate. The noble metal ion may be provided in the form of a salt,such as a noble metal chloride (e.g., palladium chloride), bromide,cyanide, fluoride, iodide, oxide hydrate, selenite, sulfate, etc. Thesalt and reducing agent may be applied in a plating solution whose pHcan be controlled as is known in the art to optimize metal deposition.Upon application, the noble metal may also be subjected to an optionalheat treatment to help it to better bond with the metal substrate. Whilethe exact temperature of heat treatment may vary depending on the noblemetal employed, it is typically high enough to allow bonding between thenoble metal and surface of the metal substrate, but not so high that itmight cause appreciable loss of material through vaporization. Forinstance, a suitable temperature range for bonding palladium to atantalum substrate may be from about 800° C. to about 1700° C., in someembodiments from about 850° C. to about 1600° C., and in someembodiments, from about 900° C. to about 1200° C. If desired, heattreatment may occur under vacuum or in the presence of an inert gas(e.g., argon, nitrogen, etc.) to inhibit the oxidation of the metalsubstrate.

ii. Conductive Polymer Layer

As noted, the conductive coating of the present invention also containsa conductive polymer layer. Although a portion of the conductive polymerlayer may be in direct contact with the metal substrate as describedabove, it is nevertheless considered to overlie the precoat layer to theextent that the precoat layer is positioned between at least a portionof the conductive polymer layer and the metal substrate.

The conductive polymer layer is formed by anodic electrochemicalpolymerization of a colloidal suspension containing a precursor monomer.The colloidal suspension may be in the form of a macroemulsion,microemulsion, solution, etc. depending on the particular nature of thecomponents of the suspension. Regardless, the suspension generallycontains a solvent that serves as a continuous phase within which theprecursor monomer is dispersed. Any of a variety of different solventsmay be employed in the colloidal suspension, such as alcohols, glycols,water, etc. In one particular embodiment, the colloidal suspension isaqueous in nature. Solvents (e.g., water) may constitute from about 50wt. % to about 99 wt. %, in some embodiments from about 70 wt. % toabout 98 wt. % and in some embodiments, from about 80 wt. % to about 95wt. %. The remaining components of the colloidal suspension (e.g.,precursor monomers, surfactants, and sulfonic acids) may likewiseconstitute from about 1 wt. % to about 50 wt. %, in some embodimentsfrom about 2 wt. % to about 30 wt. % and in some embodiments, from about5 wt. % to about 20 wt. % of the colloidal suspension.

Any of a variety of precursor monomers may be employed in the colloidalsuspension that are capable of being polymerized to form a conductivepolymer layer. Specific examples of such monomers include, for instance,pyrroles (e.g., pyrrole, alkylpyrroles, etc.), thiophenes (e.g.,3,4-ethylenedioxythiophene), anilines (e.g., alkylanilines, such asmethylaniline, and alkoxyanilines, such as methoxyaniline), as well asderivatives and combinations thereof. A single monomer may be employedto form a homopolymer, or two or more monomers may be employed to form acopolymer. In one particular embodiment, for example, a thiophenederivative monomer may be employed that has the following generalstructure:

wherein,

T is O or S;

D is an optionally substituted C₁ to C₅ alkylene radical (e.g.,methylene, ethylene, n-propylene, n-butylene, n-pentylene, etc.);

R₇ is independently selected from a linear or branched, optionallysubstituted C₁ to C₁₈ alkyl radical (e.g., methyl, ethyl, n- oriso-propyl, n-, iso-, sec- or tert-butyl, n-pentyl, 1-methylbutyl,2-methylbutyl, 3-methylbutyl, 1-ethylpropyl, 1,1-dimethylpropyl,1,2-dimethylpropyl, 2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl,2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl,n-tetradecyl, n-hexadecyl, n-octadecyl, etc.); optionally substituted C₅to C₁₂ cycloalkyl radical (e.g., cyclopentyl, cyclohexyl, cycloheptyl,cyclooctyl, cyclononyl cyclodecyl, etc.); optionally substituted C₆ toC₁₄ aryl radical (e.g., phenyl, naphthyl, etc.); optionally substitutedC₇ to C₁₈ aralkyl radical (e.g., benzyl, o-, m-, p-tolyl, 2,3-, 2,4-,2,5-, 2-6, 3-4-, 3,5-xylyl, mesityl, etc.); optionally substituted C₁ toC₄ hydroxyalkyl radical, or hydroxyl radical; and

q is an integer from 0 to 8, in some embodiments, from 0 to 2, and inone embodiment, 0. It should be understood that the R₇ group(s) may bebonded to one or more of the carbon atoms of the ring system.

Examples of substituents for the radicals “D” or “R₇” include, forinstance, hydroxyl, alkyl, cycloalkyl, aryl, aralkyl, alkoxy, halogen,ether, thioether, disulphide, sulfoxide, sulfone, sulfonate, amino,aldehyde, keto, carboxylic acid ester, carboxylic acid, carbonate,carboxylate, cyano, alkylsilane and alkoxysilane groups, carboxylamidegroups, and so forth. Particularly suitable thiophene monomers are thosein which “D” is an optionally substituted C₂ to C₃ alkylene radical. Forinstance, optionally substituted 3,4-alkylenedioxythiophenes may beemployed that have the general structure:

wherein, R₇ and q are as defined above. In one particular embodiment,“q” is 0 such that the monomer is 3,4-ethylenedioxythiophene. Onecommercially suitable example of 3,4-ethylenedioxthiophene is availablefrom Heraeus Clevios under the designation Clevios™ M. Of course, asnoted above, derivatives of 3,4-ethylenedioxythiophene may also beemployed. The derivatives may be made up of identical or differentmonomer units and used in pure form and in a mixture with one anotherand/or with the monomers. For instance, suitable derivatives of3,4-ethylenedioxythiophene include those having the following generalstructure:

where,

y is from 1 to 10, in some embodiments from 1 to 5, in some embodiments,from 1 to 3, and in some embodiments, from 1 to 2 (e.g., 2); and

R is a linear or branched, optionally substituted C₁ to C₁₈ alkylradical (e.g., methyl, ethyl, n- or iso-propyl, n-, iso-, sec- ortert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl,1-ethylpropyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl,n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl,n-octadecyl, etc.); optionally substituted C₅ to C₁₂ cycloalkyl radical(e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononylcyclodecyl, etc.); optionally substituted C₆ to C₁₄ aryl radical (e.g.,phenyl, naphthyl, etc.); optionally substituted C₇ to C₁₈ aralkylradical (e.g., benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-,3,5-xylyl, mesityl, etc.); optionally substituted C₁ to C₄ hydroxyalkylradical; hydroxyl radical; or a combination thereof. Examples ofsubstituents for the radicals “R” include, for instance, hydroxyl,alkyl, cycloalkyl, aryl, aralkyl, alkoxy, halogen, ether, thioether,disulphide, sulfoxide, sulfone, sulfonate, amino, aldehyde, keto,carboxylic acid ester, carboxylic acid, carbonate, carboxylate, cyano,alkylsilane and alkoxysilane groups, carboxylamide groups, and so forth.Particular examples of such polymers include hydroxyethylatedpoly(3,4-ethylenedioxythiophene) (y is 2 and R is OH) is andhydroxymethylated poly(3,4-ethylenedioxthiophene) (y is 1 and R is OH).It should be understood that other “R” group(s) may also be bonded toone or more other the carbon atoms of the ring system.

Suitable pyrrole monomers may likewise include those having thefollowing general structure:

wherein,

R₁ is independently selected from hydrogen, alkyl, alkenyl, alkoxy,alkanoyl, alkylhio, aryloxy, alkylthioalkyl, alkylaryl, arylalkyl,amino, alkylamino, dialkylamino, aryl, alkylsulfinyl, alkoxyalkyl,alkylsulfonyl, arylthio, arylsulfinyl, alkoxycarbonyl, arylsulfonyl,acrylic acid, phosphoric acid, phosphonic acid, halogen, nitro, cyano,hydroxyl, epoxy, silane, siloxane, alcohol, benzyl, carboxylate, ether,amidosulfonate, ether carboxylate, ether sulfonate, ester sulfonate, andurethane; or both R₁ groups together may form an alkylene or alkenylenechain completing a 3, 4, 5, 6, or 7-membered aromatic or alicyclic ring,which ring may optionally include one or more divalent nitrogen, sulfuror oxygen atoms; and

R₂ is hydrogen, alkyl, alkenyl, aryl, alkanoyl, alkylthioalkyl,alkylaryl, arylalkyl, amino, epoxy, silane, siloxane, alcohol, benzyl,carboxylate, ether, ether carboxylate, ether sulfonate, ester sulfonate,and urethane. In one particular embodiment, both R₁ and R₂ are hydrogen.Other suitable pyrroles may include 3-alkylpyrroles, such as3-hexylpyrrole; 3,4-dialkylpyrroles, such as 3,4-dihexylpyrrole;3-alkoxypyrroles, such as 3-methoxypyrrole; and 3,4-dialkoxypyrroles,such as 3,4-dimethoxypyrrole.

The total concentration of monomers employed in the colloidal suspensionmay vary, but is typically from about 0.1 wt. % to about 15 wt. %, insome embodiments from about 0.4 wt. % to about 10 wt. %, and in someembodiments, from about 0.5 wt. % to about 5 wt. % by weight of thecolloidal suspension.

A surfactant may also be employed in the colloidal suspension to formmicelles that lead to an increase in solubility, forming amacroscopically or microscopically homogenous distribution of thesemicelles and the precursor monomer. The surfactant may be ionic (e.g.,anionic, cationic, or zwitterionic) or nonionic in nature. The ionicsurfactant may, for instance, be an anionic surfactant, such as asulfonate (e.g., alkyl arylene sulfonates, α-olefin sulfonates, β-alkoxyalkane sulfonates, alkylauryl sulfonates, alkyl monoglyceridesulfonates, alkyl ether sulfonates, etc.); sulfate (e.g., alkylsulfates, alkyl aryl sulfates, alkyl ether sulfates, alkyl monoglyceridesulfates, etc.); sulfosuccinate; sarcosinate; etc., as well asderivatives, salts, polymers, and/or mixtures of the foregoing.Particular examples of ionic surfactants include, but are not limitedto, C₈-C₃₀ alkyl sulfates, C₈-C₃₀ alkyl ether sulfates having one or twomoles of ethoxylation, C₈-C₃₀ alkoyl sarcosinates, C₈-C₃₀ sulfoacetates,C₈-C₃₀ sulfosuccinates, C₈-C₃₀ alkyl diphenyl oxide disulfonates, C₈-C₃₀alkyl carbonates, C₈-C₃₀ arylene sulfonates, etc. The C₈-C₃₀ alkyl groupmay be straight chain (e.g., dodecyl) or branched (e.g., 2-ethylhexyl).The cation of the ionic surfactant may be a proton, alkali metal (e.g.,sodium or potassium), ammonium, C₁-C₄ alkylammonium (e.g., mono-, di-,tri-), or C₁-C₃ alkanolammonium (e.g., mono-, di-, tri). In oneparticular embodiment, for example, the anionic surfactant may be analkyl benzenesulfonate having the following general structure:

wherein,

R₁ is an alkyl group having from 8 to 30 carbon atoms, in someembodiments from 9 to 20, and in some embodiments, from 10 to 14 (e.g.,12) carbon atoms; and

M is cation, such as hydrogen, a metal (e.g., sodium, potassium,lithium, etc.), ammonia (NH₄ ⁺), etc. Comparable compounds with anaphthalene nucleus also can be used to form alkylnaphthalenesulfonates. Without intending to be limited by theory, it isbelieved that such alkyl arylene sulfonates are particularly effectivein enhancing the surface coverage of the colloidal suspension on thesubstrate while also facilitating charge transport.

Of course, in addition to or in lieu of an anionic surfactant, cationicsurfactants and/or zwitterionic surfactants may also be employed.Examples of cationic surfactants may include amino acids, alkyl aminesalts, quaternary ammonium salts, pyridium salts, etc. For instance,suitable alkyl amine salts may include salts of primary or secondaryamines having 3 to 22 carbon atoms, and carboxylic acid having 1 to 22carbon atoms or inorganic mineral acid, such as dodecylamine acetatesalt, dodecylamine hydrochloride salt, dodecylamine stearate salt,dodecylamine sulfonate, dimethylamine stearate salt, etc. In certainembodiments, such cationic surfactants may be formed in situ within thecolloidal suspension through the addition of an amine (e.g.,dodecylamine) and an acid, such as a sulfonic acid described below(e.g., toluene sulfonic acid).

Nonionic surfactants may also be employed. Such surfactants typicallyhave a hydrophobic base, such as a long chain alkyl group or analkylated aryl group, and a hydrophilic chain containing a certainnumber (e.g., 1 to about 30) of ethoxy and/or propoxy moieties. Althoughnot necessarily required, nonionic surfactants having a certainhydrophilic/lipophilic balance (“HLB”) value may help improve thestability of the colloidal suspension. The HLB index is well known inthe art and is a scale that measures the balance between the hydrophilicand lipophilic solution tendencies of a compound with lower numbersrepresenting highly lipophilic tendencies and the higher numbersrepresenting highly hydrophilic tendencies. In some embodiments of thepresent invention, the HLB value of the nonionic surfactant is fromabout 5 to about 20, in some embodiments from about 10 to about 19 andin some embodiments, from about 11 to about 18. If desired, two or moresurfactants may be employed that have HLB values either below or abovethe desired value, but together have an average HLB value within thedesired range.

Suitable nonionic surfactants may include, for instance, polyoxyethylenechains as hydrophilic groups, polyglycerol fatty acid esters,polyglycerol fatty alcohol ethers, sucrose fatty acid esters, andhydrocarbyl polyglycosides. In one embodiment, the nonionic surfactantincludes polyoxyethylene chains as hydrophilic groups and is selectedfrom the group of polyoxyethylene fatty acid esters, polyoxyethylenefatty alcohol ethers, polyoxyethylene sorbitol anhydride fatty acidesters, polyoxyethylene glycerol mono fatty acid esters, polyoxyethylenehydrogenated castor oil and polyoxyethylene hydrogenated castor oil monofatty acid esters, etc., as well as combinations thereof. Particularlysuitable are polyoxyethylene fatty alcohol ethers in which the fattyalcohol forming the polyoxyethylene fatty alcohol ether is saturated orunsaturated, and has 8 to 22 carbon atoms (e.g., 8 to 14), and thepolyoxyethylene structure moiety contains on average 4 to 60 ethyleneoxide repeating units (e.g., 4 to 12). Examples of such surfactantsinclude polyoxyethylene octyl ethers (e.g., polyoxyethylene-5 octylether), polyoxyethylene decyl ethers, polyoxyethylene lauryl ethers(e.g., polyoxyethylene-8 lauryl ether or polyoxyethylene-10-laurylether), polyoxyethylene myristyl ethers, polyoxyethylene palmitylethers, polyoxyethylene isostearyl ethers, polyoxyethylene stearylethers, polyoxyethylene oleyl ethers, polyoxyethylene behenyl ethers,etc.

Regardless of its particular form, the surfactant can facilitate theformation of a colloidal suspension of precursor monomer droplets.Without intending to be limited by theory, it is believed that suchdroplets can result in the formation of relatively small polymer unitson the surface of the cathode substrate during anodic electrochemicalpolymerization. Such smaller polymer units can, in turn, result in acoating that is substantially uniform with excellent surface coverage.The size of the droplets depends in part on the nature of thesuspension. “Microemulsions”, for instance, may contains droplets havingan average diameter of about 5 micrometers or less, in some embodimentsabout 4 micrometers or less, in some embodiments from about 10nanometers to about 2 micrometers, and in some embodiments, from about20 nanometers to about 1 micrometer.

“Macroemulsions” may likewise contain droplets having a size of fromabout 5 to about 100 micrometers, and in some embodiments, from about 10to about 80 micrometers. The term “diameter” can refer to the“hydrodynamic equivalent diameter” of a particle as determined usingknown techniques, such as photon correlation spectroscopy, dynamic lightscattering, quasi-elastic light scattering, etc. These methods aregenerally based on the correlation of particle size with diffusionproperties of particles obtained from Brownian motion measurements.Brownian motion is the random movement of the particles due tobombardment by the solvent molecules that surround the particles. Thelarger the particle, the more slowly the Brownian motion will be.Velocity is defined by the translational diffusion coefficient. Themeasured particle size value thus relates to how the particle moveswithin a liquid and is termed the “hydrodynamic diameter.” Variousparticle size analyzers may be employed to measure the diameter in thismanner. One particular example is a Cordouan VASCO 3 Particle SizeAnalyzer.

To help achieve the desired improvement in the surface coverage of theprecursor monomer, it is also generally desired that the concentrationof the surfactant is selectively controlled within a certain rangerelative to the precursor monomer. For example, the ratio of the weightof surfactants to the weight of precursor monomers within the colloidalsuspension may be from about 0.5 to about 1.5, in some embodiments fromabout 0.6 to about 1.4, and in some embodiments, from about 0.8 to about1.2. Surfactants may, for instance, constitute from about 0.2 wt. % toabout 10 wt. %, in some embodiments from about 0.5 wt. % to about 8 wt.%, and in some embodiments, from about 1 wt. % to about 5 wt. % of thecolloidal suspension. The total concentration of monomers employed inthe colloidal suspension may also be from about 0.1 wt. % to about 15wt. %, in some embodiments from about 0.4 wt. % to about 10 wt. %, andin some embodiments, from about 0.5 wt. % to about 5 wt. % by weight ofthe colloidal suspension.

The colloidal suspension may also contain a sulfonic acid that can actas a secondary dopant to provide excess charge to the conductive polymerand stabilize its conductivity. Such acids may, for example, result in acolloidal suspension that has an electrical conductivity of from about 1to about 100 milliSiemens per centimeter (“mS/cm”), in some embodimentsfrom about 5 to about 60 mS/cm, and in some embodiments, from about 15to about 50 mS/cm, determined at a temperature of 23° C. using any knownelectric conductivity meter (e.g., Oakton Con Series 11). The nature ofthe sulfonic acid, as wells as its relative concentration, may also beselectively controlled so that the pH level of the colloidal suspensionis within a range of from about 2.0 to about 8.5, in some embodimentsfrom about 3.0 to about 8.0, and in some embodiments, from about 5.0 toabout 7.5. For example, the ratio of the weight of sulfonic acids to theweight of precursor monomers within the colloidal suspension is fromabout 0.2 to about 1.2, in some embodiments from about 0.4 to about 1.1,and in some embodiments, from about 0.6 to about 1.0. Likewise, theratio of the weight of sulfonic acids to the weight of surfactantswithin the colloidal suspension is from about 0.2 to about 1.2, in someembodiments from about 0.3 to about 0.9, and in some embodiments, fromabout 0.4 to about 0.8.

The sulfonic acid is typically a low molecular weight organic-basedmonosulfonic acid, disulfonic acid, or trisulfonic acid. Specificexamples of such acids include, for instance, alkylsulfonic acids (e.g.,2-acrylamide-2-methylpropanesulfonic acid, etc.); arylene sulfonicacids, such as benzenesulfonic acids (e.g., phenolsulfonic acid,styrenesulfonic acid, p-toluenesulfonic acid, dodecylbenzenesulfonicacid, etc.) and naphthalenesulfonic acids (e.g., 1-naphthalenesulfonicacid, 2-naphthalenesulfonic acid, 1,3-naphthalenedisulfonic acid,1,3,6-naphthalenetrisulfonic acid, 6-ethyl-1-naphthalenesulfonic acid,etc.); anthraquinonesulfonic acids (e.g., anthraquinone-1-sulfonic acid,anthraquinone-2-sulfonic acid, anthraquinone-2,6-disulfonic acid,2-methylanthraquinone-6-sulfonic acid, etc.); camphorsulfonic acids, aswell as derivatives and mixtures thereof. Arylene sulfonic acids areparticularly suitable for use in the colloidal suspension, such as1-naphthalenesulfonic acid, 2-naphthalenesulfonic acid, and/orp-toluenesulfonic acid.

It should be understood that the term “sulfonic acid” as used hereinalso encompass salts of acids, such as those noted above, which candissociate in an aqueous solution, such as sodium salts, lithium salts,potassium salts, etc. In one embodiment, for example, the sulfonic acidmay be a sodium or potassium salt of 1-naphthalenesulfonic acid,2-naphthalenesulfonic acid, and/or p-toluenesulfonic acid.

In addition to the components noted above, the colloidal suspension mayalso contain various other additives. For example, a defoaming agent maybe employed in certain embodiments to reduce the degree of foam producedby the surfactant during the anodic electrochemical polymerization.Suitable defoaming agents may include, for instance, oils, esters,ethers, glycols, polysiloxanes, long chain oxygenated hydrocarbons(e.g., C₆ to C₁₂ alcohols), etc., as well as mixtures thereof.Particularly suitable defoaming agents are long chain oxygenatedhydrocarbons, such as octanol, decanol and polyethylene glycols. Whenemployed, such defoaming agents may constitute from about 0.01 wt. % toabout 5 wt. %, in some embodiments from about 0.05 wt. % to about 4 wt.%, and in some embodiments, from about 0.1 wt. % to about 2 wt. % of thecolloidal suspension. Besides defoaming agents, a wide variety of otheradditives may also be employed in the colloidal suspension.

To apply the colloidal suspension to the metal substrate, any of avariety of suitable application techniques may be employed, such asscreen-printing, dipping, electrophoretic coating, spraying, etc.Regardless of how it is applied, the monomer within the colloidalsuspension is anodically electrochemically-polymerized to form aconductive polymer layer on the surface of the metal substrate. In oneembodiment, for example, the metal substrate is dipped into a bathcontaining the colloidal suspension of the present invention. A pair ofelectrodes may be disposed within the bath for electrolysis. Oneelectrode may be connected to the positive terminal of a power sourceand also in contact with the metal substrate. The other electrode may beconnected to the negative terminal of the power source and an additionalinert metal. During operation, the power source supplies a current feedto the electrodes in the electrochemical cell, thereby inducingelectrolysis of the electrolyte and oxidative polymerization of themonomer in the colloidal suspension onto the metal substrate. Anodicelectrochemical polymerization is generally performed at ambienttemperature to ensure that the colloidal suspension does not phaseseparate. For example, the colloidal suspension may be kept at atemperature of from about 15° C. to about 80° C., in some embodimentsfrom about 20° C. to about 75° C., and in some embodiments, from about25° C. to about 50° C. The amount of time in which the metal substrateis in contact with the colloidal suspension during anodicelectrochemical polymerization may vary. For example, the metalsubstrate may be dipped into such a solution for a period of timeranging from about 10 seconds to about 10 minutes.

The resulting conductive polymer layer includes conductive polymer(s)that are typically π-conjugated chains of aromatic heterocyclic unitsand have increased electrical conductivity after oxidation. Because theconductive polymer is generally semi-crystalline or amorphous, it candissipate and/or absorb the heat associated with the high voltage. Thisin turn prevents fluid electrolyte phase transitions (from liquid togas) at the interface. The conductive polymer will somewhat swellthrough absorption of some volume of fluid electrolyte. Examples of suchπ-conjugated conductive polymers that can be formed in accordance withthe present invention include polyheterocycles (e.g., polypyrroles,polythiophenes, polyanilines, etc.), polyacetylenes, poly-p-phenylenes,polyphenolates, and so forth. In one particular embodiment, thesubstituted polythiophene has the following general structure:

wherein,

T, D, R₇, and q are defined above; and

n is from 1 to 1,000, in some embodiments from 2 to 500, and in someembodiments, from 4 to 350. Particularly suitable thiophene polymers arethose in which “D” is an optionally substituted C₂ to C₃ alkyleneradical. For instance, the polymer may be optionally substitutedpoly(3,4-ethylenedioxythiophene), which has the following generalstructure:

If desired, multiple polymerization steps may be repeated until thedesired thickness of the coating is achieved. In such cases, theadditional layer(s) may be polymerized using the technique and colloidalsuspension of the present invention, or using other methods, such aschemical polymerization. The additional layer(s) (e.g., chemicallypolymerized layer) may be disposed directly on the pre-coat layer orover the electro-polymerized layer. Regardless, the total targetthickness of the conductive polymer layer(s) may generally varydepending on the desired properties of the capacitor. Typically, theresulting conductive polymer layer(s) have a thickness of from about 0.2micrometers (“μm”) to about 50 μm, in some embodiments from about 0.5 μmto about 20 μm, and in some embodiments, from about 1 μm to about 5 μm.It should be understood that the thickness of the layers are notnecessarily the same at all locations on the substrate. Nevertheless,the average thickness on the substrate generally falls within the rangesnoted above.

iii. Other Layers

If desired, the conductive coating may also contain other types oflayers for a variety of different purposes. For example, the conductivecoating may contain a hydrogen protection layer, which can furtherabsorb and dissipate hydrogen radicals. In certain embodiments, theprecoat layer may overlie the hydrogen protection layer. In suchembodiments, the hydrogen protection layer may be positioned between themetal substrate and the precoat layer and also positioned adjacent tothe metal substrate.

When employed, the hydrogen protection layer may include a plurality ofrelatively small, high surface area agglomerates that are sinteredtogether so that they form a more integral and robust coating. Althoughnot necessarily required, the agglomerates may also be sintered to themetal substrate so that the protection layer remains more readilyadhered thereto. The shape of the agglomerates may vary, such asspherical, nodular, flake, etc. Typically, the agglomerates are selectedto have a high specific charge to help increase the cathode capacitance,such as about 70,000 microFarads*Volts per gram (“pPV/g”) or more, insome embodiments about 80,000 μF*V/g or more, in some embodiments about90,000 μF*V/g or more, in some embodiments about 100,000 μF*V/g or more,and in some embodiments, about 120,000 to about 350,000 μF*V/g. Examplesof valve metal compositions for forming such agglomerates include valvemetals, such as tantalum, niobium, aluminum, hafnium, titanium, alloysthereof, etc., as well as oxides thereof (e.g., niobium oxide), nitridesthereof, and so forth. In a preferred embodiment, the compositioncontains tantalum.

The agglomerates may, for example, have an aggregate D50 size of about100 micrometers or less, in some embodiments from about 1 to about 80micrometers, and in some embodiments, from about 10 to about 70micrometers, wherein the term “D50 size” generally means that at least50% of the agglomerates having a size within the denoted range asdetermined by sieve analysis. The particle size distribution of theagglomerates may also be relatively narrow. For example, no more thanabout 5%, in some embodiments no more than about 2%, and in someembodiments, no more than about 1% of the agglomerates having a sizegreater than 150 micrometers.

In addition to having a small aggregate size within a controlled range,the primary particle size of the agglomerates may also be relativelysmall. For example, the average primary particle size of theagglomerates may be from about 5 nanometers to about 20 micrometers, insome embodiments from about 10 nanometers to about 10 micrometers, insome embodiments from about 15 nanometers to about 5 micrometers, and insome embodiments, from about 20 nanometers to about 800 nanometers. Theagglomerates may likewise have a relatively high specific surface area,such as about 1.2 m²/g or more, in some embodiments about 1.5 m²/g ormore, and in some embodiments, from about 2.0 to about 8.0 m²/g. The“specific surface area” may be determined using a variety of techniquesknown in the art, such as by the physical gas adsorption (B.E.T.) methodof Bruanauer, Emmet, and Teller, Journal of American Chemical Society,Vol. 60, 1938, p. 309, with nitrogen as the adsorption gas.

The agglomerates may be formed using a variety of techniques. Aprecursor tantalum powder, for instance, may be formed by reducing atantalum salt (e.g., potassium fluotantalate (K₂TaF₇), sodiumfluotantalate (Na₂TaF₇), tantalum pentachloride (TaCl₅), etc.) with areducing agent (e.g., hydrogen, sodium, potassium, magnesium, calcium,etc.). Such powders may be agglomerated in a variety of ways, such asthrough one or multiple heat treatment steps at a temperature of fromabout 700° C. to about 1400° C., in some embodiments from about 750° C.to about 1200° C., and in some embodiments, from about 800° C. to about1100° C. Heat treatment may occur in an inert or reducing atmosphere.For example, heat treatment may occur in an atmosphere containinghydrogen or a hydrogen-releasing compound (e.g., ammonium chloride,calcium hydride, magnesium hydride, etc.) to partially sinter the powderand decrease the content of impurities (e.g., fluorine). If desired,agglomeration may also be performed in the presence of a gettermaterial, such as magnesium. After thermal treatment, the highlyreactive coarse agglomerates may be passivated by gradual admission ofair. Other suitable agglomeration techniques are also described in U.S.Pat. No. 6,576,038 to Rao; U.S. Pat. No. 6,238,456 to Wolf, et al.; U.S.Pat. No. 5,954,856 to Pathare et al.; U.S. Pat. No. 5,082,491 to Rerat;U.S. Pat. No. 4,555,268 to Getz; U.S. Pat. No. 4,483,819 to Albrecht, etal.; U.S. Pat. No. 4,441,927 to Getz, et al.; and U.S. Pat. No.4,017,302 to Bates, et al.

The desired size and/or shape of the agglomerates may be achieved bysimply controlling various processing parameters as is known in the art,such as the parameters relating to powder formation (e.g., reductionprocess) and/or agglomeration (e.g., temperature, atmosphere, etc.).Milling techniques may also be employed to grind a precursor powder tothe desired size. Any of a variety of milling techniques may be utilizedto achieve the desired particle characteristics. For example, the powdermay initially be dispersed in a fluid medium (e.g., ethanol, methanol,fluorinated fluid, etc.) to form a slurry. The slurry may then becombined with a grinding media (e.g., metal balls, such as tantalum) ina mill. The number of grinding media may generally vary depending on thesize of the mill, such as from about 100 to about 2000, and in someembodiments from about 600 to about 1000. The starting powder, the fluidmedium, and grinding media may be combined in any proportion. Forexample, the ratio of the starting powder to the grinding media may befrom about 1:5 to about 1:50. Likewise, the ratio of the volume of thefluid medium to the combined volume of the starting powder may be fromabout 0.5:1 to about 3:1, in some embodiments from about 0.5:1 to about2:1, and in some embodiments, from about 0.5:1 to about 1:1. Someexamples of mills that may be used in the present invention aredescribed in U.S. Pat. Nos. 5,522,558; 5,232,169; 6,126,097; and6,145,765. Milling may occur for any predetermined amount of time neededto achieve the target size. For example, the milling time may range fromabout 30 minutes to about 40 hours, in some embodiments, from about 1hour to about 20 hours, and in some embodiments, from about 5 hours toabout 15 hours. Milling may be conducted at any desired temperature,including at room temperature or an elevated temperature. After milling,the fluid medium may be separated or removed from the powder, such as byair-drying, heating, filtering, evaporating, etc.

Various other conventional treatments may also be employed in thepresent invention to improve the properties of the powder. For example,in certain embodiments, the agglomerates may be doped with sinterretardants in the presence of a dopant, such as aqueous acids (e.g.,phosphoric acid). The amount of the dopant added depends in part on thesurface area of the powder, but is typically present in an amount of nomore than about 200 parts per million (“ppm”). The dopant may be addedprior to, during, and/or subsequent to any heat treatment step(s). Theagglomerates may also be subjected to one or more deoxidation treatmentsto improve ductility. For example, the agglomerates may be exposed to agetter material (e.g., magnesium), such as described in U.S. Pat. No.4,960,471. The temperature at which deoxidation occurs may vary, buttypically ranges from about 700° C. to about 1600° C., in someembodiments from about 750° C. to about 1200° C., and in someembodiments, from about 800° C. to about 1000° C. The total time ofdeoxidation treatment(s) may range from about 20 minutes to about 3hours. Deoxidation also preferably occurs in an inert atmosphere (e.g.,argon). Upon completion of the deoxidation treatment(s), the magnesiumor other getter material typically vaporizes and forms a precipitate onthe cold wall of the furnace. To ensure removal of the getter material,however, the agglomerates may be subjected to one or more acid leachingsteps, such as with nitric acid, hydrofluoric acid, etc.

Certain additional components may also be incorporated into the powder.For example, the powder may be optionally mixed with a binder to ensurethat the agglomerates adequately adhere to each other when applied tothe substrate. Suitable binders may include, for instance, poly(vinylbutyral); poly(vinyl acetate); poly(vinyl alcohol); poly(vinylpyrrolidone); cellulosic polymers, such as carboxymethylcellulose,methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, andmethylhydroxyethyl cellulose; atactic polypropylene, polyethylene;polyethylene glycol (e.g., Carbowax™ from Dow Chemical Co.);polystyrene, poly(butadiene/styrene); polyamides, polyimides, andpolyacrylamides, high molecular weight polyethers; copolymers ofethylene oxide and propylene oxide; fluoropolymers, such aspolytetrafluoroethylene, polyvinylidene fluoride, and fluoro-olefincopolymers; acrylic polymers, such as sodium polyacrylate, poly(loweralkyl acrylates), poly(lower alkyl methacrylates) and copolymers oflower alkyl acrylates and methacrylates; and fatty acids and waxes, suchas stearic and other soapy fatty acids, vegetable wax, microwaxes(purified paraffins), etc. The binder may be dissolved and dispersed ina solvent. Exemplary solvents may include water, alcohols, and so forth.

Any of a variety of techniques may generally be employed to apply theagglomerates to the metal substrate in accordance with the presentinvention, such as heat treating, thermal sintering, sputtering,screen-printing, dipping, electrophoretic coating, electron beamdeposition, spraying, roller pressing, brushing, doctor blade casting,and vacuum deposition. Excess agglomerates may be removed, for instance,by inverting the substrate. Upon application, the agglomerates mayoptionally be heated to remove any binder/lubricant present. Heating mayoccur, for instance, at a temperature of from about 40° C. to about 800°C. Once applied, one or more hydrogen protection layers of theagglomerates are typically formed on the substrate surface. Thethickness of a protection layer may vary depending on the size andconfiguration of the metal substrate, anode, etc. Generally speaking, aprotection layer may have a thickness of from about 1 to about 1000micrometers, in some embodiments from about 2 to about 800 micrometers,and in some embodiments, from about 5 to about 500 micrometers. Theextent to which the agglomerates cover the metal substrate may alsovary. For example, in certain embodiments, the protection layer may besubstantially continuous such that they agglomerates cover a substantialportion, if not all of the surface of the substrate. In yet otherembodiments, the protection layer may be discontinuous such that theagglomerates are applied in, for example, a spaced-apart fashion overthe surface so that they form “island-like” structures. Regardless ofhow the protection layer is formed, the agglomerates may be sintered sothat a bond forms between the particles and optionally the metalsubstrate. For example, sintering may be conducted at a temperature offrom about 800° C. to about 2000° C., in some embodiments from about1200° C. to about 1800° C., and in some embodiments, from about 1500° C.to about 1700° C., for a time of from about 5 minutes to about 100minutes, and in some embodiments, from about 10 minutes to about 50minutes. This may occur in one or more steps. If desired, sintering mayoccur in a reducing atmosphere, such as in a vacuum, inert gas,hydrogen, etc. The reducing atmosphere may be at a pressure of fromabout 10 Torr to about 2000 Torr, in some embodiments from about 100Torr to about 1000 Torr, and in some embodiments, from about 100 Torr toabout 930 Torr. Mixtures of hydrogen and other gases (e.g., argon ornitrogen) may also be employed.

Regardless of the total number of layers employed, the total targetthickness of the conductive coating may generally vary depending on thedesired properties of the capacitor. Typically, the conductive coatinghas a thickness of from about 0.5 micrometers (“μm”) to about 300 μm, insome embodiments from about 1 μm to about 250 μm, and in someembodiments, from about 10 μm to about 150 μm. It should be understoodthat the thickness of the coating is not necessarily the same at alllocations on the substrate. Nevertheless, the average thickness of thecoating on the substrate generally falls within the ranges noted above.

II. Anode

The anode of the electrolytic capacitor includes a porous body that maybe formed from a valve metal composition, such as described above. Inone embodiment, for example, the anode is formed from a tantalum powder.The powder may contain agglomerates having any of a variety of shapes,such as nodular, angular, flake, etc., as well as mixtures thereof. Incertain cases, the anode may be formed from a powder having a highspecific charge. That is, the powder may have a specific charge of about70,000 microFarads*Volts per gram (“μF*V/g”) or more, in someembodiments about 80,000 μF*V/g or more, in some embodiments about90,000 μF*V/g or more, in some embodiments about 100,000 μF*V/g or more,and in some embodiments, from about 120,000 to about 350,000 μF*V/g. Ofcourse, although powders of a high specific charge are normally desired,it is not necessarily a requirement. In certain embodiments, forexample, powders having a specific charge of less than about 70,000microFarads*Volts per gram (“μF*V/g”), in some embodiments about 2,000μF*V/g to about 65,000 μF*V/g, and in some embodiments, from about 5,000to about 50,000 μF*V/g.

Once formed, the resulting powder may be compacted using anyconventional powder press mold. For example, the press mold may be asingle station compaction press using a die and one or multiple punches.Alternatively, anvil-type compaction press molds may be used that useonly a die and single lower punch. Single station compaction press moldsare available in several basic types, such as cam, toggle/knuckle andeccentric/crank presses with varying capabilities, such as singleaction, double action, floating die, movable platen, opposed ram, screw,impact, hot pressing, coining or sizing. The powder may be compactedaround an anode lead (e.g., tantalum wire). It should be furtherappreciated that the anode lead may alternatively be attached (e.g.,welded) to the anode body subsequent to pressing and/or sintering of theanode body. If desired, any binder may be removed after compression,such as by heating the formed pellet under vacuum at a certaintemperature (e.g., from about 150° C. to about 500° C.) for severalminutes. Alternatively, binder may also be removed by contacting thepellet with an aqueous solution, such as described in U.S. Pat. No.6,197,252 to Bishop, et al. Regardless, the pressed anode body issintered to form a porous, integral mass. The sintering conditions maybe within the ranges noted above.

Referring to FIGS. 2-4, for example, one embodiment of an anode 20 isshown that contains a porous, sintered body 22 having at least onesidewall 24 positioned between a proximal end 34 and an opposing distalend 36. The cross-sectional shape of the proximal end 34 and/or thedistal end 36 may generally vary based on the desired shape of the anodebody 22. In this particular embodiment, for example, both ends 34 and 36have a circular cross-sectional shape such that the anode body 22 isgenerally cylindrical. Other suitable shapes may include, for instance,square, rectangular, triangular, hexagonal, octagonal, heptagonal,pentagonal, trapezoidal, elliptical, star, sinusoidal, etc. The anodebody 22 also has a length in the longitudinal direction “z” definedbetween the ends 34 and 36, and a width in the “x” direction and depthin the “y” direction. In the illustrated embodiment, the width and depthare both defined between the sidewalls 24. Although by no means arequirement, the length of the anode body 22 is typically greater thanits width and/or depth. For example, in certain embodiments, the ratioof the length to both the width and depth may be from about 1 to about30, in some embodiments from about 1.1 to about 10, and in someembodiments, from about 1.5 to about 5. The length of the anode 20 may,for example, range from about 0.5 to about 100 millimeters, in someembodiments from about 1 to about 60 millimeters, and in someembodiments, from about 5 to about 30 millimeters. The width of theanode body 22 may range from about 0.5 to about 50 millimeters, in someembodiments from about 1 to about 40 millimeters, and in someembodiments, from about 4 to about 30 millimeters. Likewise, the depthof the anode body 22 may range from about 0.5 to about 50 millimeters,in some embodiments from about 1 to about 40 millimeters, and in someembodiments, from about 4 to about 30 millimeters. Of course, when theanode body is cylindrical in nature, its width and depth will be thesame.

In certain embodiments, at least one longitudinally extending channel isrecessed into the anode body. Such channels may be formed duringpressing as would be known to those skilled in the art. For example, thepress mold may contain one or more longitudinal indentations thatcorrespond to the desired shape of the channels. In this manner, thepowder is compressed around the indentations so that when removed fromthe mold, the resulting anode body contains longitudinal channels atthose areas where the longitudinal indentations were located in themold.

The channels may have a relatively high aspect ratio (length divided bywidth), such as about 2 or more, in some embodiments about 5 or more, insome embodiments from about 10 to about 200, in some embodiments fromabout 15 to about 150, in some embodiments from about 20 to about 100,and in some embodiments, from about 30 to about 60. Such channels cansignificantly increase the outer surface area of the anode, which mayenhance the degree to which the anode can dissipate heat and increasethe likelihood that the anodizing electrolyte will pass into the poresof the anode body during anodic oxidation. Referring again to FIGS. 2-4,for instance, the anode body 22 may contain channels 28 that arerecessed into the sidewall 24. The channels 28 are “longitudinallyextending” in the sense that they possess a length in the longitudinaldirection “z” of the anode body 22. However, while the channels 28 ofFIGS. 2-4 are substantially parallel with the longitudinal direction,this is by no means a requirement. For example, other suitableembodiments may include one or more longitudinally extending channelsthat are in the form of a spiral, helix, etc., which are not parallelwith the longitudinal of the anode body. The number of suchlongitudinally extending channels may vary, but is typically from 1 to20, in some embodiments from 2 to 15, and in some embodiments, from 4 to10. When multiple channels are employed, it is generally desired thatthey are distributed symmetrically and equidistant about a centerlongitudinal axis of the anode, although this is by no means arequirement. In FIGS. 2-4, for example, the depicted anode body 22contains five (5) separate channels 28. FIG. 5, on the other hand, showsan alternative embodiment in which six (6) separate channels 228 areemployed. In each of the particular embodiments, however, the channelsare distributed in a generally symmetric manner about the longitudinalcenter “C” of the anode body,

At least a portion of the channels 28 may have a relatively high aspectratio (length divided by width). The length “L₁” (FIG. 3) of thechannels 28 may, for example, range from about 0.5 to about 100millimeters, in some embodiments from about 1 to about 60 millimeters,and in some embodiments, from about 5 to about 30 millimeters. The width“W₁” of the channels 28 (FIGS. 3 and 4) may likewise range from about0.01 to about 20 millimeters, in some embodiments from about 0.02 toabout 15 millimeters, in some embodiments from about 0.05 to about 4millimeters, and in some embodiments, from about 0.1 to about 2millimeters.

The channels 28 shown in FIGS. 2-4 extend in the longitudinal direction“L” along the entire length of the anode body 22 and intersect both theproximal end 34 and the distal end 36. It should be understood, however,that one or more channels may also extend along only a portion of theanode body length so that they intersect only one end of the anode body,or so that they do not intersect either end.

The extent to which the channels are recessed into the anode body may beselectively controlled to achieve a balance between increased surfaceand integrity of the anode structure. That is, if the depth of thechannels is too great, it may be difficult to press the anode into aphysically strong structure. Likewise, if the depth is too small, thedesired benefits may not be achieved. Thus, in most embodiments, thechannels are recessed so that they extend in a direction that is fromabout 2% to about 60%, in some embodiments from about 5% to about 50%,and in some embodiments, from about 10% to about 45% of the thickness ofthe anode body in the same direction. Referring again to FIG. 3, forexample, one of the channels 28 is shown as extending in a direction“T.” In this embodiment, the length of the channel 28 in the direction“T” divided by the thickness of the porous body 22 in the direction “T”,multiplied by 100, is within the percentages referenced above.

Of course, the depth of each of the channels need not be the same.Referring to FIG. 5, for example, one embodiment of an anode 220 isshown that contains first channels 228 and second channels 229. In thisparticular embodiment, the first channels 228 extend into the anode bodyto a greater degree than the second channels 229. One of the firstchannels 228 may, for example, extend in a direction “T₁” that is fromabout 15% to about 60%, in some embodiments from about 20% to about 50%,and in some embodiments, from about 25% to about 45% of the thickness ofthe anode body in the same direction. Likewise, one of the secondchannels 229 may extend in a direction “T₂” that is from about 2% toabout 40%, in some embodiments from about 5% to about 35%, and in someembodiments, from about 10% to about 25% of the anode body in the samedirection. Such a configuration can effectively combine the benefits ofthe deeper channels (e.g., greater surface area) with those of theshallower channels (e.g., greater physical integrity). In suchembodiments, the number of deeper channels may be from 1 to 10, in someembodiments from 2 to 6, and in some embodiments, from 2 to 4, and thenumber of shallower channels may likewise be from 1 to 10, in someembodiments from 2 to 6, and in some embodiments, from 2 to 4.

Typically, the anode also contains an anode lead wire that helps connectthe anode to the terminations of the resulting capacitor. The lead wiremay be formed from any electrically conductive material, such astantalum, niobium, nickel, aluminum, hafnium, titanium, etc., as well asoxides and/or nitrides of thereof. Although not necessarily required, itis often desired that the lead wire extend in the same longitudinaldirection as the channels. In the embodiment of FIGS. 2-4, for example,an anode lead wire 30 extends in the longitudinal “z” direction from theproximal end 34 of the anode body 22. Electrical contact with the anode20 may be accomplished by in a variety of ways, such as by coupling thelead wire 30 using resistance or laser welding. Alternatively, the leadwire 30 may be embedded into the anode body during its formation (e.g.,prior to sintering).

Once formed, the porous anode body is anodically oxidized (“anodized”)so that a dielectric layer is formed over and/or within the anode body.For example, a tantalum (Ta) anode body may be anodized to tantalumpentoxide (Ta₂O₅). Anodization may be performed using electrolytesolutions such as those described above. Typically, the voltage at whichanodic oxidation of the anode occurs ranges from about 4 to about 250 V,and in some embodiments, from about 9 to about 200 V, and in someembodiments, from about 20 to about 150 V.

III. Working Electrolyte

The working electrolyte is in electrical communication with the metalsubstrate and anode. The electrolyte is a fluid that may be impregnatedwithin the anode, or it may be added to the capacitor at a later stageof production. The fluid electrolyte generally uniformly wets thedielectric on the anode. Various suitable electrolytes are described inU.S. Pat. Nos. 5,369,547 and 6,594,140 to Evans, et al. Typically, theelectrolyte is ionically conductive in that has an electricalconductivity of from about 0.1 to about 20 Siemens per centimeter(“S/cm”), in some embodiments from about 0.2 to about 15 S/cm, and insome embodiments, from about 0.5 to about 10 S/cm, determined at atemperature of about 23° C. using any known electric conductivity meter(e.g., Oakton Con Series 11). The fluid electrolyte is generally in theform of a liquid, such as a solution (e.g., aqueous or non-aqueous),colloidal suspension, gel, etc. For example, the electrolyte may be anaqueous solution of an acid (e.g., sulfuric acid, phosphoric acid, ornitric acid), base (e.g., potassium hydroxide), or salt (e.g., ammoniumsalt, such as a nitrate), as well any other suitable electrolyte knownin the art, such as a salt dissolved in an organic solvent (e.g.,ammonium salt dissolved in a glycol-based solution). Various otherelectrolytes are described in U.S. Pat. Nos. 5,369,547 and 6,594,140 toEvans, et al.

The desired ionic conductivity may be achieved by selecting ioniccompound(s) (e.g., acids, bases, salts, and so forth) within certainconcentration ranges. In one particular embodiment, salts of weakorganic acids may be effective in achieving the desired conductivity ofthe electrolyte. The cation of the salt may include monatomic cations,such as alkali metals (e.g., Li⁺, Na⁺, K⁺, Rb⁺, or Cs⁺), alkaline earthmetals (e.g., Be²⁺, Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺), transition metals (e.g.,Ag⁺, Fe²⁺, Fe³⁺, etc.), as well as polyatomic cations, such as NH₄ ⁺.The monovalent ammonium (NH₄ ⁺), sodium (K⁺), and lithium (Li⁺) areparticularly suitable cations for use in the present invention. Theorganic acid used to form the anion of the salt may be “weak” in thesense that it typically has a first acid dissociation constant (pK_(a1))of about 0 to about 11, in some embodiments about 1 to about 10, and insome embodiments, from about 2 to about 10, determined at about 23° C.Any suitable weak organic acids may be used in the present invention,such as carboxylic acids, such as acrylic acid, methacrylic acid,malonic acid, succinic acid, salicylic acid, sulfosalicylic acid, adipicacid, maleic acid, malic acid, oleic acid, gallic acid, tartaric acid(e.g., dextotartaric acid, mesotartaric acid, etc.), citric acid, formicacid, acetic acid, glycolic acid, oxalic acid, propionic acid, phthalicacid, isophthalic acid, glutaric acid, gluconic acid, lactic acid,aspartic acid, glutaminic acid, itaconic acid, trifluoroacetic acid,barbituric acid, cinnamic acid, benzoic acid, 4-hydroxybenzoic acid,aminobenzoic acid, etc.; blends thereof, and so forth. Polyprotic acids(e.g., diprotic, triprotic, etc.) are particularly desirable for use informing the salt, such as adipic acid (pK_(a1) of 4.43 and pK_(a2) of5.41), α-tartaric acid (pK_(a1) of 2.98 and pK_(a2) of 4.34),meso-tartaric acid (pK_(a1) of 3.22 and pK_(a2) of 4.82), oxalic acid(pK_(a1) of 1.23 and pK_(a2) of 4.19), lactic acid (pK_(a1) of 3.13,pK_(a2) of 4.76, and pK_(a3) of 6.40), etc.

While the actual amounts may vary depending on the particular saltemployed, its solubility in the solvent(s) used in the electrolyte, andthe presence of other components, such weak organic acid salts aretypically present in the electrolyte in an amount of from about 0.1 toabout 25 wt. %, in some embodiments from about 0.2 to about 20 wt. %, insome embodiments from about 0.3 to about 15 wt. %, and in someembodiments, from about 0.5 to about 5 wt. %.

The electrolyte is typically aqueous in that it contains an aqueoussolvent, such as water (e.g., deionized water). For example, water(e.g., deionized water) may constitute from about 20 wt. % to about 95wt. %, in some embodiments from about 30 wt. % to about 90 wt. %, and insome embodiments, from about 40 wt. % to about 85 wt. % of theelectrolyte. A secondary solvent may also be employed to form a solventmixture. Suitable secondary solvents may include, for instance, glycols(e.g., ethylene glycol, propylene glycol, butylene glycol, triethyleneglycol, hexylene glycol, polyethylene glycols, ethoxydiglycol,dipropyleneglycol, etc.); glycol ethers (e.g., methyl glycol ether,ethyl glycol ether, isopropyl glycol ether, etc.); alcohols (e.g.,methanol, ethanol, n-propanol, iso-propanol, and butanol); ketones(e.g., acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters(e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate,methoxypropyl acetate, ethylene carbonate, propylene carbonate, etc.);amides (e.g., dimethylformamide, dimethylacetamide,dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones);sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane);and so forth. Such solvent mixtures typically contain water in an amountfrom about 40 wt. % to about 80 wt. %, in some embodiments from about 50wt. % to about 75 wt. %, and in some embodiments, from about 55 wt. % toabout 70 wt. % and secondary solvent(s) in an amount from about 20 wt. %to about 60 wt. %, in some embodiments from about 25 wt. % to about 50wt. %, and in some embodiments, from about 30 wt. % to about 45 wt. %.The secondary solvent(s) may, for example, constitute from about 5 wt. %to about 45 wt. %, in some embodiments from about 10 wt. % to about 40wt. %, and in some embodiments, from about 15 wt. % to about 35 wt. % ofthe electrolyte.

If desired, the electrolyte may be relatively neutral and have a pH offrom about 4.5 to about 8.0, in some embodiments from about 5.0 to about7.5, and in some embodiments, from about 5.5 to about 7.0. One or morepH adjusters (e.g., acids, bases, etc.) may be employed to help achievethe desired pH. In one embodiment, an acid is employed to lower the pHto the desired range. Suitable acids include, for instance, organicacids such as described above; inorganic acids, such as hydrochloricacid, nitric acid, sulfuric acid, phosphoric acid, polyphosphoric acid,boric acid, boronic acid, etc.; and mixtures thereof. Although the totalconcentration of pH adjusters may vary, they are typically present in anamount of from about 0.01 wt. % to about 10 wt. %, in some embodimentsfrom about 0.05 wt. % to about 5 wt. %, and in some embodiments, fromabout 0.1 wt. % to about 2 wt. % of the electrolyte.

The electrolyte may also contain other components that help improve theelectrical performance of the capacitor. For instance, a depolarizer maybe employed in the electrolyte to help inhibit the evolution of hydrogengas at the cathode of the electrolytic capacitor, which could otherwisecause the capacitor to bulge and eventually fail. When employed, thedepolarizer normally constitutes from about 1 to about 500 parts permillion (“ppm”), in some embodiments from about 10 to about 200 ppm, andin some embodiments, from about 20 to about 150 ppm of the electrolyte.Suitable depolarizers may include nitroaromatic compounds, such as2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2-nitrobenzonic acid,3-nitrobenzonic acid, 4-nitrobenzonic acid, 2-nitroace tophenone,3-nitroacetophenone, 4-nitroacetophenone, 2-nitroanisole,3-nitroanisole, 4-nitroanisole, 2-nitrobenzaldehyde,3-nitrobenzaldehyde, 4-nitrobenzaldehyde, 2-nitrobenzyl alcohol,3-nitrobenzyl alcohol, 4-nitrobenzyl alcohol, 2-nitrophthalic acid,3-nitrophthalic acid, 4-nitrophthalic acid, and so forth. Particularlysuitable nitroaromatic depolarizers for use in the present invention arenitrobenzoic acids, anhydrides or salts thereof, substituted with one ormore alkyl groups (e.g., methyl, ethyl, propyl, butyl, etc.). Specificexamples of such alkyl-substituted nitrobenzoic compounds include, forinstance, 2-methyl-3-nitrobenzoic acid; 2-methyl-6-nitrobenzoic acid;3-methyl-2-nitrobenzoic acid; 3-methyl-4-nitrobenzoic acid;3-methyl-6-nitrobenzoic acid; 4-methyl-3-nitrobenzoic acid; anhydridesor salts thereof; and so forth.

IV. Sealing Assembly

The anode and working electrolyte of the capacitor are generallypositioned within the interior of the casing. In certain embodiments,the casing may define an opening that is closed by a sealing assembly.The sealing assembly includes, for instance, a hermetic seal that isgenerally formed from an insulative material, such as glass. If desired,a conductive tube may be employed that has an orifice of a size andshape sufficient to accommodate the anode lead. The conductive tube istypically formed from a metal, such as tantalum, niobium, aluminum,nickel, hafnium, titanium, copper, silver, steel (e.g., stainless),alloys thereof (e.g., electrically conductive oxides), compositesthereof (e.g., metal coated with electrically conductive oxide), and soforth. In such embodiments, the conductive tube may pass through a borewithin the hermetic seal so that it is electrically insulated.

The arrangement of the sealing assembly within a capacitor is notcritical and may vary as would be appreciated by those skilled in theart. Referring to FIG. 1, for example, one particular embodiment of acapacitor 100 is shown. As shown, the capacitor 100 contains a casing 12having a sidewall 14 and a lower wall 16. Multiple sidewalls may beemployed in such embodiments where the casing is not cylindrical. Aconductive coating 17 is also formed on at least a portion of the casing12 as discussed above. For example, the conductive coating 17 may bedisposed on the inner surfaces of the sidewall 14 and the lower wall 16.An anode 20 is also positioned within an interior 11 of the casing 12.An anode lead 30 may extend from the anode 20 in a longitudinaldirection through a conductive tube 56.

The capacitor 100 also contains a sealing assembly 50. In thisparticular embodiment, the sealing assembly covers an opening 59 definedbetween crimped portions of the casing 12. Alternatively, however, a lidmay be provided as is known in the art that defines the opening. In anyevent, a hermetic seal 54 (e.g., glass-to-metal seal) is positionedwithin the opening 59 in the illustrated embodiment that defines a borethrough which the conductive tube 56 and the anode lead 30 can pass. Thesealing assembly 50 also includes a barrier seal 70, such as a sealformed from an elastomeric material. The seal 70 may have a generallycylindrical shape and contain a bore coaxially located therein throughwhich the conductive tube 56 and the anode lead 42 can pass. In thismanner, the barrier seal 70 can cover at least a portion of the lowersurface of the hermetic seal 54 to limit its contact with anyelectrolyte. If desired, the barrier seal 70 may cover a substantialportion of the lower surface of the hermetic seal 54. By “substantialportion”, it is generally meant that the seal covers about 80% or moreof the surface, in some embodiments about 90% or more of the surface,and in some embodiments, about 100% of the surface. As shown in FIG. 1,the barrier seal 70 also typically covers at least a portion of theconductive tube 56.

In addition to the sealing assembly discussed above, the capacitor ofthe present invention may also contain one or more secondary seals. Forexample, additional gaskets or bobbin may be employed that are formedfrom non-elastomeric insulative materials, such aspolytetrafluorethylene (“PTFE”). In one embodiment, for example, abobbin 90 may be positioned between the anode 20 and the barrier seal70. Elastomeric rings 92 may also be employed, such as adjacent to thesidewall 14 of the casing 12. The elastomeric rings 92 may be formedfrom a high-temperature elastomer, such as described above, or fromanother type of elastomeric material. Also, if desired, a support may beprovided in contact with the anode to help ensure that it remainsmechanically stable during use. The support may be from an insulativematerial, such as polytetrafluorethylene (“PTFE”). One example of such asupport is shown in FIG. 2 as element 55, which is positioned adjacentto and in contact with the lower surface of the anode 20. An externalpositive lead 82 may likewise be connected to the anode lead 30 at anend of the conductive tube 56 via a weld joint 80 and external negativelead 83 may be connected to the lower wall 16 of the casing 12.

Regardless of the particular configuration, the resulting capacitor ofthe present invention may exhibit excellent electrical properties. Forexample, the capacitor may exhibit a high energy density. Energy densityis generally determined according to the equation E=½*CV², where C isthe capacitance in farads (F) and V is the working voltage of capacitorin volts (V). The energy density may, for example, be about 2.0 joulesper cubic centimeter (J/cm³) or more, in some embodiments about 3.0J/cm³, in some embodiments from about 4.0 J/cm³ to about 10.0 J/cm³, andin some embodiments, from about 4.5 to about 8.0 J/cm³. The capacitancemay likewise be about 1 milliFarad per square centimeter (“mF/cm²”) ormore, in some embodiments about 2 mF/cm² or more, in some embodimentsfrom about 5 to about 50 mF/cm², and in some embodiments, from about 8to about 20 mF/cm², as determined at an operating frequency of 120 Hz.The equivalent series resistance (“ESR”) may also be less than about 500milliohms, in some embodiments less than about 400 milliohms, in someembodiments less than about 300 milliohms, and in some embodiments, fromabout 1 to about 200 milliohms, as determined at a frequency of 120 Hz.In addition, the leakage current, which generally refers to the currentflowing from one conductor to an adjacent conductor through aninsulator, can be maintained at relatively low levels. For example, thenumerical value of the normalized leakage current of a capacitor of thepresent invention is, in some embodiments, less than about 1 μA/μF*V, insome embodiments less than about 0.5 μA/μF*V, and in some embodiments,less than about 0.1 μA/μF*V, where μA is microamps and μF*V is theproduct of the capacitance and the rated voltage. Notably, due to theunique nature of the conductive coating of the present invention, thepresent inventors have discovered that such capacitance, ESR, and/ornormalized leakage current values may even be maintained after aging athigh temperatures. For example, the values may be maintained attemperatures ranging from about 50° C. to about 200° C. (e.g., 85° C.)for a substantial period of time, such as about 100 hours or more, insome embodiments from about 300 hours to about 2500 hours, and in someembodiments, from about 400 hours to about 1500 hours (e.g., 500 hours,600 hours, 700 hours, 800 hours, 900 hours, 1000 hours, 1100 hours, or1200 hours).

The electrolytic capacitor of the present invention may be used invarious applications, including but not limited to micro-inverters;micro-UPS devices; medical devices, such as implantable defibrillators,pacemakers, cardioverters, neural stimulators, drug administeringdevices, etc.; automotive applications; military applications, such asRADAR systems; consumer electronics, such as radios, televisions, etc.;and so forth.

The present invention may be better understood by reference to thefollowing examples.

Test Procedures Equivalent Series Resistance (“ESR”)

Equivalence series resistance may be measured using a Keithley 3330Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 voltpeak to peak sinusoidal signal. The operating frequency may be 120 Hzand the temperature may be 23° C.±2° C.

Capacitance (“CAP”)

The capacitance may be measured using a Keithley 3330 Precision LCZmeter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak topeak sinusoidal signal. The operating frequency may be 120 Hz and thetemperature may be 23° C.±2° C.

Leakage Current:

Leakage current (“DCL”) may be measured using a leakage test meter at atemperature of 85° C.±2° C. and at the rated voltage after a minimum of60 seconds.

Materials Employed

The following materials were employed in the examples.

PdCl₂=Palladium(11) chloride (Merck);

EDT=3,4-ethylenedioxythiophene (Hereaus);

pTSA=p-toluene sulfonic acid, sodium salt; and

POE-10-LE=polyoxyethylene-10-fauryl ether (or decaethylene glycolmonodecyl ether).

Example 1

Initially, 10 samples of cylindrical tantalum cans were sandblasted witha JetStreem Blaster II (SCM System, Inc.) for about 20 seconds. Thesamples were degreased in water in an ultrasonic bath and dried at atemperature of 85° C. for 5 minutes. An amount of 0.3 g of PdCl₂ wasadded to 25 ml of 1.0M hydrochloric acid in a 50 ml flask. The can wasfilled with the previously prepared PdCl₄ ²⁻ aqueous acidic solution andplaced in a copper receptacle connecting the can to the negative pole ofa power supply. A Pt wire was electrically connected to the positivepole of the power supply and inserted into the can and PdCl₄ ²⁻ aqueousacidic solution. Electro-deposition was performed for about 15 minutesusing a constant current setting of 50 mA to form a structure ofpalladium. The cans were then rinsed in water to remove reactionby-products and dried at 85° C. for 5 minutes. 8 to 9 mg of palladiumwas deposited onto each tantalum can. A precursor solution wasthereafter applied to the palladium-deposited surface that contained 4 gof ethanol (Sigma-Aldrich), 0.1 g of methylpyrrolidone (Sigma-Aldrich),1 g of EDT, and 10 g of 40% butanol solution of iron(III)-p-toluenesulfonate (Heraeus). The tantalum cans were filled to the control levelwith the polymerization precursor solution for 5 minutes and were thenput into a drying oven at 85° C. for 15 minutes. The resulting structureof poly(3,4-ethylenedioxythiophene) was washed in methanol to removereaction by-products for 5 minutes and the tantalum cans were put into adrying oven at 85° C. for 5 minutes. This polymerization cycle wasrepeated 4 times. 4-5 mg of poly(3,4-ethylenedioxythiophene) wasdeposited onto each palladium-coated tantalum can. FIG. 6 is amicrophotograph of the resulting tantalum/palladium/PEDT structure.

Next, 10 samples of a cylindrical anode were pressed from tantalumpowder (70,000 μF*V/g) with six symmetrical longitudinally extendingchannels recessed into the anode body. The anodes were sintered at atemperature of 1440° C. for 10 minutes and anodized to 75V (the singleanode exhibited a capacitance of 3000 μF at a frequency of 120 Hz) wereadded into the previously prepared cans. The electrolyte was a 5.0 Maqueous solution of sulfuric acid (specific gravity of 1.26 g/cm³). Thecomponents were then assembled into a wet capacitor.

Example 2

Wet capacitors were formed in the manner described in Example 1, exceptthat the palladium precoat was not employed. FIG. 7 is a microphotographof the resulting tantalum/PEDT structure.

The capacitors of Examples 1 and 2 were then tested in the mannerdescribed above. The measurements were taken and then repeated atdifferent times for 2000 hours of total life testing at an applied ratedvoltage of 50V. The results are set forth below.

Example 1 Example 2 time CAP ESR DCL CAP ESR DCL [h] [uF] [Ohm] [uA][uF] [Ohm] [uA] 0 2710 0.273 587.30 2700 0.283 904.24 100 2460 0.321 —50 7.340 — 500 1906 0.494 — 8 10.182 — 1000 1235 1.232 57.36 3 29.31653.74 1500 1365 1.322 — 3 34.646 2000 1325 1.356 112.35 2 25.555 82.50

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

What is claimed is:
 1. A wet electrolytic capacitor comprising: an anodeformed from an anodically oxidized sintered porous body; a fluidicworking electrolyte; and a casing within which the anode and workingelectrolyte are positioned, wherein the casing contains a conductivecoating disposed on a surface of a metal substrate, the conductivecoating containing: a discontinuous precoat layer that contains aplurality of discrete projections of a conductive material depositedover the surface of the metal substrate in a spaced-apart fashion sothat the projections cover from about 5% to about 80% of the surface ofthe metal substrate; and a conductive polymer layer that overlies thediscontinuous precoat layer, wherein the conductive polymer layer isformed by electrolytic polymerization of a colloidal suspension thatincludes a precursor monomer.
 2. The wet electrolytic capacitor of claim1, wherein the conductive material is a noble metal.
 3. The wetelectrolytic capacitor of claim 2, wherein the noble metal is palladium.4. The wet electrolytic capacitor of claim 1, wherein the average sizeof the projections is from about 50 nanometers to about 500 micrometers.5. The wet electrolytic capacitor of claim 1, wherein the precursormonomer includes a thiophene and the conductive polymer layer includes apolythiophene.
 6. The wet electrolytic capacitor of claim 5, wherein thepolythiophene is an optionally substitutedpoly(3,4-ethylenedioxythiophene) having the following general structure:

wherein, R₇ is a linear or branched, optionally substituted C₁ to C₁₈alkyl radical; optionally substituted C₅ to C₁₂ cycloalkyl radical;optionally substituted C₆ to C₁₄ aryl radical; optionally substituted C₇to C₁₈ aralkyl radical; optionally substituted C₁ to C₄ hydroxyalkylradical, or hydroxyl radical; q is an integer from 0 to 8; and n is from2 to 5,000.
 7. The wet electrolytic capacitor of claim 1, wherein thecolloidal suspension includes a surfactant and a sulfonic acid.
 8. Thewet electrolytic capacitor of claim 7, wherein the surfactant is analkyl arylene sulfonate.
 9. The wet electrolytic capacitor of claim 7,wherein the sulfonic acid is an arylene sulfonic acid.
 10. The wetelectrolytic capacitor of claim 1, wherein the conductive polymer layeris free of high energy iron radicals.
 11. The wet electrolytic capacitorof claim 1, wherein the metal substrate is formed from tantalum.
 12. Thewet electrolytic capacitor of claim 1, wherein the conductive coatingfurther includes a hydrogen protection layer that is positioned betweenthe metal substrate and the precoat layer, wherein the hydrogenprotection layer contains a plurality of sintered valve metalagglomerates formed from a valve metal composition that are sintered tothe metal substrate.
 13. The wet electrolytic capacitor of claim 1,wherein a dielectric layer is formed on the metal substrate that ispositioned between the substrate and the conductive coating.
 14. The wetelectrolytic capacitor of claim 1, wherein the anode includes tantalum.15. The wet electrolytic capacitor of claim 1, wherein a plurality oflongitudinally extending channels are recessed into the porous body. 16.The wet electrolytic capacitor of claim 1, wherein the workingelectrolyte is a liquid.
 17. The wet electrolytic capacitor of claim 1,wherein the working electrolyte is an aqueous solution containingsulfuric acid.
 18. The wet electrolytic capacitor of claim 1, whereinthe working electrolyte has a pH value of from about 4.5 to about 8.0.19. The wet electrolytic capacitor of claim 1, wherein the casingdefines an opening and a sidewall surrounding an interior, and wherein asealing assembly covers the opening.
 20. A method for forming a casingof a wet electrolytic capacitor, the method comprising: forming adiscontinuous precoat layer on a surface of a metal substrate, theprecoat layer containing a plurality of discrete projections of aconductive material deposited over the surface of the metal substrate ina spaced-apart fashion so that the projections cover from about 5% toabout 80% of the surface of the metal substrate; applying a colloidalsuspension to the metal substrate, wherein the colloidal suspensioncomprises a precursor monomer; placing an electrode in contact with themetal substrate; and supplying a current feed to the electrode to induceelectrolysis and oxidative polymerization of the precursor monomer,thereby forming a conductive polymer layer that overlies the precoatlayer.
 21. The method of claim 20, wherein the precursor monomerincludes 3,4-alkylenedioxythiophene or a derivative thereof.
 22. Themethod of claim 20, wherein the colloidal suspension includes asurfactant and sulfonic acid.
 23. The method of claim 22, wherein thesurfactant is an alkyl arylene sulfonate.
 24. The method of claim 22,wherein the sulfonic acid is an arylene sulfonic acid.
 25. The method ofclaim 20, wherein the colloidal suspension is free of high energy ironradicals.
 26. The method of claim 20, wherein the precoat layer isformed by electroless plating.
 27. The method of claim 20, wherein theprecoat layer is subjected to a heat treatment at a temperature of fromabout 800° C. to about 1700° C.
 28. The method of claim 20, wherein theprecoat layer includes palladium.
 29. The method of claim 20, whereinthe metal substrate is formed from tantalum.
 30. The method of claim 20,wherein a plurality of valve metal agglomerates are sintered to thesurface of the metal substrate before the precoat layer is formedthereon.